1. Field of the Invention
The present invention relates to a measuring head intended for measuring diameters of cylindrical parts, in particular for controlling a machine for grinding outer cylindrical surfaces. The head is of the type comprising a support, an upper block and a lower block which are attached flexibly to said support and arranged to be able to move away from or nearer to one another while remaining aligned with one another, a transducer for measuring the distance between the blocks, and two measuring projections for resting on the cylindrical part to be measured, the projections being fast with said upper block and said lower block respectively.
2. Description of the Prior Art
A measuring head of this kind is known, for example, from German patent application published on Nov. 29th 1956 under Number Sch 17 246 Ib/67a. If reference is made to FIG. 1 of the mentioned document, there will be recognised a support to which an upper block and a lower block are flexibly attached by means of the spring leaves. The distance between the blocks is measured by a transducer located at the top of the drawing. Each of the blocks is equipped with a measuring projection and the diameter of the cylindrical part to be measured is sensed by the ends of the measuring projections or contact elements, which are formed of hard metal or diamond. In this way, the reduction in the diameter of the part caused by the operation of a grinding wheel is converted into an electric signal issuing from the pick-up. This signal can be used to display the size or only the reduction in size or for stopping the operation of the grinding wheel automatically when a predetermined size has been reached. Depending on the embodiment, the transducer may be inductive, optical or capacitive.
It will be noted that, for a given cylindrical part, the measuring range extends only over some tenths of a millimeter, since what is concerned here are grinding operations in which only little material is removed. However, the measuring head must be capable of adaptation to parts to be machined which have very different nominal dimensions. It is therefore necessary to be able to adjust the distance between the projections within a considerable bracket, for example of some millimetres to several centimetres. In the document mentioned, this problem has been solved by mounting the measuring projection on a rail in the form of a dovetail which can slide on another rail of opposite form constituting part of the block and provided with a rack. A toothed wheel controlled by an adjusting knob allows this adjustment. Once the said adjustment has been made, the entire arrangement is locked by means of a screw.
An arrangement very similar to that described above is shown in the document No. FR-A-1,268,074, in which we again find two blocks attached flexibly to a support. Here, however, the blocks are disposed side by side instead of being superposed one with respect to the other as in the preceding document. The document No. FR-A-1,268,074 makes use of a guide which bears a plate through which a screw can turn which is engaged in a slide capable of sliding on the guide and of being clamped thereto with the aid of a screw. The slide bears a projection on which is fixed a block which serves to support rigidly the rod bearing a point constituting a measuring projection.
As regards the adjustment of the projections, the constructions which have just been described have the drawback of requiring parts which are complicated and, therefore, costly. It must also be pointed out that the necessary time required by this adjustment is relatively considerable. Finally, on clamping the locking screw, there is always the risk of putting the projections out of adjustment.
In order to alleviate these disadvantages, there has already been proposed a measuring head in which the two projections are articulated something like a pair of dividers. Here, the axes of rotation of the projections are parallel to the axis of the cylinder to be ground. Once the points are in contact with the cylinder, the projections are locked. If this system has the advantage of permitting more rapid adjustment, it has the drawback of not sensing accurately the diameter of the part to be machined, since the measuring contact elements are no longer parallel, but form an angle between them. This leads to errors of measurement which are due to the inevitable variations in position existing between the measuring head and the part to be machined.
The document No. CH-A-626,991 describes, for its part, a rotary transducer for measuring position by contact by means of cranks coupled to devices measuring the angle of rotation of the said cranks. This system has the great disadvantage of needing pick-ups which permit measurement over an extensive range. Such pick-ups are costly and difficult to adjust.
In almost similar manner, the document U.S. Pat. No. 3,391,463 proposes a feeler consisting of an arm and a rod in the form of a roller. The arm is mounted to pivot on an axis. It is a question in this case of the conversion of a rotary movement into a linear movement. Examination of the document makes it obvious that the measuring range is extremely limited, which is not the case in the present invention, as will be seen hereinafter.